1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to a sheet conveying apparatus and an image forming apparatus with a structure for reforming a curl generated on a sheet.
2. Description of the Related Art
Up to now, in an image forming apparatus such as a copying machine and a printer for forming an image by an electrophotographic process, a toner image which has been formed in an image forming part is transferred onto a sheet fed from a sheet feeding part, and the toner image having been transferred onto the sheet is fixed to the sheet in a fixing part.
As an example of such the fixing part, there is a fixing part employing a heat-pressure fixing system for fixing a toner image to a sheet by applying heat and pressure to the sheet at the same time by a rotary member such as a pair of rollers. In a case of adopting the fixing part employing the heat-pressure fixing system, there arises a problem in that deformation (i.e., curl) is caused on a sheet when the toner image is fixed to the sheet.
In general, as the curl generated by the fixing part, there are three types of curls, that is, a heat curl, a toner curl, and a nip curl. As shapes of the curl, hereinafter, for understanding, an upward curl is defined as a shape in which leading and trailing edges of the sheet in a conveying direction of a sheet is higher than a central portion of the sheet, while a downward curl is defined as a shape in which leading and trailing edges of the sheet is lower than the central portion of the sheet.
Here, the heat curl is generated due to a difference between an amount of water contained in a front surface of the sheet and that contained in a back surface of the sheet. When heat is applied to the sheet in the fixing part, the heat curl is generated due to an expansion and contraction difference between both surfaces of the sheet which is caused by the difference between the amount of water contained in the front surface and that contained in the back surface of the sheet. There is a case where the heat curl is more likely to be generated because the sheet is prevented from absorbing moisture due to toner carried on the surface of the sheet.
The toner curl is generated during such a process that the toner transferred onto the sheet is melted by being applied with heat in the fixing part, and is then cooled and condensed.
The nip curl is generated due to physical deformation caused when the sheet is applied with heat and pressure in a nip part of a fixing roller in the fixing part for fixing toner to a sheet by application of heat and pressure with a pair of fixing rollers.
Here, sizes of the above-mentioned three-types of curls generated are different from each other depending on a plurality of causes such as a thickness or humidity (i.e., amount of humidity absorption) of a sheet, a toner amount transferred onto the sheet, and a temperature difference between nip rollers of the fixing part. For example, in the heat curl generated due to the difference between the amount of water contained in the front surface and that contained in the back surface of the sheet, a large difference in moisture absorption is easily generated between both surfaces of the sheet under such circumstances as thick paper or high humidity, thereby generating a large curl.
The toner curl generated due to the condensed toner transferred onto the sheet is affected by an amount of contraction at a time of toner contraction, a toner carrying amount, and a difference in an amount of toner carried on both surfaces of the sheet. In addition, when rigidity of the sheet is low, the sheet is more likely to be affected by the toner curl, so thin paper is more easily curled to a large extent than thick paper.
The nip curl generated depending on the shape of a nip part of the fixing roller tends to be the upward curl in which the leading and trailing edges of the sheet become higher than the central portion thereof, because, for example, the shape of the nip part becomes a convex shape in the downward direction when an upper roller is composed of a hard roller and a lower roller is composed of a soft roller.
Thus, there is the case where the sheet is curled due to various causes when the sheet passes through the fixing part. When the curled sheet is discharged without reforming the curl, there arises such problems that the sheet is curled up on a discharge tray, or subsequent sheets cannot be stacked on the tray due to blocking of an outlet by the trailing edge of the sheet, thereby deteriorating a stacking property of a sheet.
Thus, up to now, an image forming apparatus including a sheet conveying apparatus for conveying sheets having passed through the fixing part onto the discharge tray, which is provided with a curl reforming apparatus provided downstream of the fixing part and used for reforming a curl of a sheet has been put into practical use. The curl reforming apparatus is used for reforming a shape of a sheet by curling the sheet in a direction opposite to that of a curl of the sheet already generated by the fixing part. In the present application, a direction of a curl of the sheet before being reformed is defined as “pre-curled direction”, and a direction of a curl of the sheet in which the sheet is curled to reform the curl of the sheet is defined as “curling direction”, to thereby distinguish the directions of the curls. In addition, an amount of curl in the pre-curled direction is defined as “pre-curled amount”, and an amount of curl in the curling direction in which the sheet is curled to reform the curl of the sheet is defined as “curling amount”.
As a system of the curl reforming apparatus, there are a system capable of controlling the curling amount of the sheet by using an actuator, and a system in which the curling amount is not controlled by setting the curling amount to be constant.
Here, in a case of the curl reforming apparatus which does not control the curling amount, the pre-curled direction is not fixed to one direction due to the thickness of the sheet or the toner carrying amount as described above, so there is a case where the pre-curled direction matches the curling direction by the curl reforming apparatus, which results in further curling of the sheet. In such the case, there has been a problem of, for example, curling of the sheet on the discharge tray.
Accordingly, as the curl reforming apparatus for sufficiently reforming a curl of a sheet, there has been proposed a curl reforming apparatus in which a curl forming part for curling a sheet in an upward direction is provided along with another curl forming part for curling a sheet in a downward direction, and one of the curl forming parts is selectively used based on detection by curl detecting means for detecting a curl of a sheet.
Further, as an example of the curl reforming apparatus with such the structure, there is one disclosed in Japanese Patent Application Laid-open No. 2004-026419 in which the pre-curled direction and the pre-curled amount of the sheet are detected by the curl detecting means, and the curling amount of the curl forming part selected based on the detected pre-curled direction and the pre-curled amount is adjusted, to thereby appropriately reform the curl of the sheet.
FIG. 5 is an explanatory view of a sheet conveying apparatus including such the conventional curl reforming apparatus. In FIG. 5, reference symbol FX denotes a fixing part, and reference numeral 104 denotes a curl reforming apparatus for reforming a curl of a sheet P. The fixing part FX includes an entering guide FX1, a heat roller FX 2 provided on an inner side of a fixing heat source FX4, a pressure roller FX3, and discharge rollers FX5. An outer peripheral surface of the heat roller FX2 is set such that the outer peripheral surface has a wider rubber layer as compared with the pressure roller FX3, and the rubber layer of the heat roller FX2 is deformed to a large extent in the nip part between the heat roller FX2 and the pressure roller FX3 with the nip part having a convex shape in the upward direction.
The curl reforming apparatus 104 includes three rollers 104a to 104c serving as curl reforming members, and a support disk 104d rotationally supporting the rollers 104a to 104c. A fixed guide 100 is a fixed guide composed of a pair of guide plates 100a and 100b, movable guides 101 and 102 are composed of a pair of guide plates 101a and 101b and a pair of guide plates 102a and 102b, respectively, and a fixed guide 103 is composed of a pair of guide plates 103a and 103b. 
The guide plates 101a and 101b of the movable guide 101 and the guide plates 102a and 102b of the movable guide 102 are lightweight members and are each composed of, for example, a plastic sheet or a plurality of rods. As shown in FIG. 6B, the guide plates 101a and 101b of the movable guide 101 and the guide plates 102a and 102b of the movable guide 102 are loosely connected at a connected portion Y. In addition, downstream end portions of the movable guides 101 and 102, that is, end portions of the guide plates 101a and 102a on a side closer to the fixed guide 103 are free ends so as to move within the fixed guide 103.
In the curl reforming apparatus with such the structure, the nip part between the heat roller FX2 and the pressure roller FX3 is heated, an unfixed toner image is subjected to a fixing process, and then the sheet P passes through the fixed guide 100, the movable guides 101 and 102, and the fixed guide 103.
In this case, as shown in FIG. 6A, there is a sheet P having the downward curl, in which a central portion of the sheet becomes higher than the leading and trailing edges thereof on a side bearing the fixed toner image, depending on the thickness or humidity of the sheet, the transferred toner amount, the temperature difference between the nip rollers of the fixing part, and the like. In addition, as shown in FIG. 7, there is a sheet P having the upward curl in which a central portion of the sheet becomes lower than the leading and trailing edges thereof on a side bearing the fixed toner image.
Further, for example when the sheet P before image formation is curled, a state of the curl is varied such as the curl of the sheet remains after the image formation, or the curl is reformed, and a generating level of the curl is different in each case. In addition, in the curl generated due to the fixing process, the upward curl or the downward curl may be generated depending on a paper quality, the amount of humidity absorption, and the like of the sheet P, and the generating level of the curl is also varied.
When the sheet P passes through the movable guides 101 and 102, for example, when the curl of the sheet P is the downward curl, the movable guides 101 and 102 are deformed as shown in FIG. 6A. In FIG. 6B, a variable resistor 105 constitutes the curl detecting means together with the movable guides 101 and 102.
The variable resistor 105 includes a slider 105b which moves integrally in a vertical direction with the connected portion Y, and a resistor 105a serving as displacement detecting means for detecting a position of the slider 105b. The variable resistor 105a detects the position of the slider 105b according to a resistivity of the resistor 105a. 
In the variable resistor 105, when the sheet P enters the movable guides 101 and 102, the movable guide 101 moves in the upward direction with a base 101X as an axis along with the curl of the sheet P. Then, a movement of the movable guide 101 is converted into voltage by the variable resistor 105 serving as the displacement detecting means, and the direction (i.e., downward curl) and the level of the curl of the sheet P are detected based on a voltage value.
Further, the curl reforming apparatus 104 reforms the upward curl in a state shown in FIG. 6A, and reforms the downward curl in a state shown in FIG. 7 in which the support disk 104d is rotated by 30°. In this case, the support disk 104d is rotated by a control of controlling means 106, and the controlling means 106 controls a rotational angle of the support disk 104d based on an output of the variable resistor 105.
Accordingly, with respect to the sheet P curled in the downward direction, the curl reforming apparatus 104 becomes a state shown in FIG. 6A, thereby reforming the curl of the sheet P during a conveying process of the sheet P, and with respect to the sheet P curled in the upward direction, the curl reforming apparatus 104 becomes a state shown in FIG. 7, thereby reforming the curl of the sheet P such that the curl thereof is eliminated.
However, in the conventional sheet conveying apparatus, in order to achieve the curl detecting means and the controllable curl reforming means, there has been a problem in that a size of the apparatus is increased and the number of components is increased, which increases the size of the entire image forming apparatus and costs required for the apparatus.